Microelectronic devices, such as memory devices, microprocessors, and light emitting diodes, typically include one or more semiconductor dies mounted to a substrate and encased in a plastic protective covering. Semiconductor die include functional features, such as memory cells, processor circuits, and interconnecting circuitry. Semiconductor die also typically include bond pads electrically coupled to the functional features. The bond pads are electrically connected to pins or other types of terminals that extend outside the protective covering for connecting the semiconductor die to busses, circuits, or other assemblies.
Semiconductor die manufacturers are under increasing pressure to reduce the volume occupied by the die and yet increase the capacity and/or speed of the resulting encapsulated assemblies. To meet these demands, semiconductor die manufacturers often stack multiple die on top of each other to increase the capacity or performance of the device within the limited volume on the circuit board or other element to which the dies are mounted.
The stacked semiconductor die are typically electrically connected by solder bumps or other electrical connectors that are attached to UBM structures. The UBM structures are typically formed by depositing a copper seed structure onto a wafer, forming a mask on the copper seed structure having openings aligned with bond pads on the die, plating copper onto the seed structure, and then plating one or more other materials over the copper to form UBM pillars. The top-most material of the UBM pillars is typically selected to promote wetting for subsequently forming interconnects on the top-most material. After forming the UBM pillars, the mask is removed and the exposed portions of the seed structure are removed using a suitable wet etch to form isolated UBM structures. For example, it is often desirable to form a top-most layer of palladium or gold, and then perform a wet etch that removes the remaining portion of the copper seed structure to electrically isolate the UBM structures. However, the chemistry for wet etching the copper seed structure can significantly undercut the copper to the extent that it impairs the mechanical stability of the UBM structures and the electrical connection between the UBM structures and the underlying metal interconnects (e.g., through-substrate vias (TSVs) or other interconnects). Accordingly, there is a need for innovation and improvement in UBM structures and the methods for manufacturing these features.